The present invention relates to a circuit for providing positive feedback in an emitter-follower output circuit to reduce the output impedance of the emitter follower.
The designation "emitter-follower circuit" refers herein to a transistor biasing circuit in which a voltage developed at the emitter of the transistor is made available to a load. The voltage applied to the load is referred to herein as the output voltage, and the current drawn by the load is referred to as the output current.
The output impedance of an emitter-follower is a measure of the change in the output voltage of the circuit which results from a change in the output current. It is a function of the small signal emitter resistance of the transistor, the value of any resistance between the base-emitter junction of the transistor and the output terminal, the beta of the transistor, and the equivalent impedance of the circuitry driving the base of the emitter follower. A positive output impedance causes the output voltage of the emitter-follower circuit to decrease when an output current is drawn from the emitter-follower circuit by a load.
In circuit applications where it is desired that the emitter-follower output voltage be maintained substantially constant over a range of output cirrents, non-zero output impedance undesirably limits the range of output currents over which the emitter-follower circuit can be expected to maintain a substantially constant output voltage. The output current of an emitter-follower circuit is a function of load impedance. For that reason, a positive output impedance limits the range of load impedance to which a substantially constant output voltage can be supplied.
In view of the foregoing, it would be desirable to provide a circuit for reducing the output impedance of an emitter-follower circuit such that the output voltage of the emitter-follower circuit is maintained at a substantially constant value over a wide range of output currents.
It also would be desirable to provide a circuit for reducing the output impedance of an emitter-follower circuit such that the decrease in emitter output voltage when the output current is increased is minimized.
It further would be desirable to provide a circuit for reducing the output impedance of an emitter-follower circuit to a negative value such that the emitter output voltage increases when the output current is increased.